Q&A of Prius Brakes

[DanMan32]

This may be best targeted for Rick, Kyle and any other brake engineers out there.

I have had a heated discussion with someone over PriusOnline regarding the brake system on the Prius. He was under the impression that if the accumulator system died, you would have no hydraulic brakes. Despite my posting hydraulic diagrams, including the diagram in the NCF showing that the master cylinder chambers would be allowed to directly control the front calipers, and a worded description of the failsafe state he still insists there is no physical connection from your foot to the wheel cylinders under any conditions, therefore you would lose complete hydraulic brake control once the accumulator pressure was lost.

Anyway, one component of the hydraulic system is the Stroke Simulator that is in line with one of the chambers of the master cylinder. The repair manual and NCF is vaugh on what it exactly does. At first I thought it was to apply quick pressure pulses to the hydraulic system faster than the pump/valves could act, but the hydraulic circuit is normally cut off fromt the operational part of the hydraulic brake system, as is the master cylinders. Also, the stroke simulator is on only one side of the hydraulic system. If my original supposition was correct, there would be two chambers in the stroke simulator, as there is for the master cylinder.

So, my next theory is that it simulates the somewhat cushy feel you would have in a conventional system, that you couldn't otherwise have in this system, since your input to the hydraulics is to a dead end.
Is the simulator mostly for us to have a look and feel, or is it to provide another means of electronic control of the hydraulics?
If it is for our own feedback, how does the Skid ECU control it, besides the cutoff valve? Does the ECU change the simulator's dampening characteristic?

If anyone has other questions about the brake system, that is what this thread is for.

[hdrygas]

Well when I looked at it in New Car Features there is a direct connection and that is stated as well. I need to read the chapter again I thought the Stroke Simulator had something to do with the ABS. I will reread it.

[rick57]

Getting in deep here,aren't we I will have to do some digging/research on that. I am not sure how to answer it right now. Let me get to work and do some checking

[DanMan32]

Well, I am an inquiring mind, and I am fascinated by the genius engineering and design they put into this car. I haven't read the books cover to cover, or in my case, byte to byte (I have everything in PDF), but I do believe I have a good understanding of everthing, but there's always more to figure out and learn.

Although I mentioned the heated discussion, I am very confident on my assesment of the hydraulic operation when all power is lost. I even provided this colored diagram, with description, and the guy STILL wouldn't listen. Turns out he thought the master cylinders weren't pistons but valves. Yeah, right, valves to what?

But in my 'studies' I got curious about the stroke simulator, so I thought I'd post here, since we have at least two who actually studied and worked on these cars, and not just guess and speculate.

[Frank Hudon]

Dan in the stroke simulator you'll see a pitot valve it's not on the seat under nomal braking it allows the fluid to compress the piston at the end of it and what this does is allow the brake pedal stroke sensor to send the signal to allow regen. If you jam on the brakes the pitot valve closes and the fluid pressure builds faster to the wheel cylinder pressure switches which signals the brake control ecu to apply pressure from the accumulator to the system. This is modulated by the control valves which are many, these are in turn controled by the speed sensors on the brake rotors and in turn opened and closed as needed to keep from getting wheel lockup. The vsc system is also tied into the control valves to diagonallly apply pressure to counter yaw in a skid. The fastest way to find out if there are any brakes when the pump or accumulator fail is to find a large empty parking lot pull the fuse for the pump and drive the car and see if you have brakes, if not jump on the emergency brake. It'll have brakes, NTSA wouldn't allow the car to be on a public hiway with out backup braking.

[DanMan32]

From the wiring diagram, I found only one electrical connection so that would have to be the stroke simulator the cutoff valve for failsafe mode.

Therefore, I have surmized that the stroke simulator is to pretend to be a wheel cylinder so your foot won't know the difference.
I already knew that besides the stroke position sensor, that there is a hydraulic pressure sensor detecting hydraulic pressure from the wheel cylinder. It mainly detects when you jam on the brakes, causing sudden pressure spike.

I also already knew that under normal conditions, you never have direct control of the wheel cylinders, but rather make requests to the brake system, and it applies the brakes as required.

I just found it weird that the stroke simulator was on only one side of the split hydraulic system. It would seem that the side without the simulator would become hydro-locked, preventing any movement even on the simulator side. Maybe the linkage from the pedal allows the side without the simulator to slip up to a point, until the simulator hits bottom.

[Frank Hudon]

the stroke simulator is just there to allow the pedal to depress up to the inch and a half that's required for the pedal sensor to provide the signal to the regen circuit that it needs to slow the car with regen instead of friction brake. When you jump on the pedal the pressure switch closes the pitot circuit to allow full master cylinder volume to activate the accumulator to pressure up the system.

[hdrygas]

the stroke simulator is just there to allow the pedal to depress up to the inch and a half that's required for the pedal sensor to provide the signal to the regen circuit that it needs to slow the car with regen instead of friction brake. When you jump on the pedal the pressure switch closes the pitot circuit to allow full master cylinder volume to activate the accumulator to pressure up the system.

Let me understand this. If I place little pressure on the break, as is the usual course the stroke simulator is not active. The regeneration is in full sway. If however the light goes yellow at the last minute, as it usually does in Lacy, and I put more pressure on the peddle the stroke stimulator cuts in the hydraulics to allow me to stop, all be it at a cost? Momentum converted to Evil heat. I hate it when that happens. Besides it means I may need to replace break pads at 80K. Daxx Lacy stop lights.

[Frank Hudon]

Henry it's just the opposite with slow depression of the brake pedal the stroke simulator works,brake fluid flows thru the valve depressing the piston and spring. This allows the brake pedal to go down and it operates the pedal sensor (connected to the brake pedal) to allow regen. With rapid depression the valve is closed and the piston and spring in the simulator don't compress.

[Kyle Pehrson]

I would like to thank all of you for the posts. I think Frank is right on and agree with what you said. It's not something that we have had any problems with so haven't had to do much with it since we went to Prius school when we covered it out of the NCF manual.
Thanks